Annulus

Two concentric circles form an annulus of width 10 cm. The radius of the smaller circle is 20 cm. Calculate the content area of annulus.

Correct result:

S =  1570.7963 cm²

Solution:

$$\begin{gathered} r_2=20\text{ cm } \\ m=10\text{ cm } \\ {r}_1=r_2+m=20+10=30\text{ cm } \\ {S}_1=\pi \cdot r_1^2=3.1416\cdot 30^2\doteq 2827.4334{\text{cm}}^2 \\ {S}_2=\pi \cdot r_2^2=3.1416\cdot 20^2\doteq 1256.6371{\text{cm}}^2 \\ S=S_1-S_2=2827.4334-1256.6371=1570.7963{\text{cm}}^2 \end{gathered}$$

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